Increasing the resilience of uninterruptible power supplies (UPS)
often entails installation of a configuration known as Parallel UPS when
the outputs of two or more modules (capable of parallel operation) are
connected to supply the load via a common ac busbar. A
group of parallel UPS modules is referred to as an uninterruptible
power supply system. The two basic parallel configurations are known as
Parallel-redundancy and Parallel-capacity. Parallel-redundant UPS
systems increase resilience.
Resilience of parallel-redundant
uninterruptible power supplies can be further enhanced using dual input
supplies whereby the same UPS system is supplied from separate rectifier
and static switch supplies. Most installations rely on common mains
power supplies feeding both the UPS and static bypass but it creates a
single-point-of-failure in the design. Should an upstream
circuit-breaker trip due to a fault, the rectifier and bypass no longer
have a source of ac power. The use of dual input supplies from separately derived sources (even separate substations) removes this issue.
Typically,
transformer-based UPS have a dual input facility as standard but in
transformerless models it is a factory-fit option as the rectifier and
bypass, in this case, rely on a common neutral.
Transformer-based UPS
The
dual input option in a transformer-based UPS can be selected at
installation by simply removing a linking connector from its input
terminal. The UPS can be powered from two separate ac supply
sources because in this type of module the rectifier and bypass are
independent of each other. A typical transformer-based uninterruptible
power supply has a rectifier with a three-phase input (delta) and bypass
supply that may have either a three-phase or single-phase plus neutral
input. Some UPS of this type can also operate without a neutral
connection.
In a
transformerless UPS the rectifier and bypass supplies require a common
neutral connection, made within the module itself. This type of UPS can
be installed with a dual input but with supplies derived from the same
source, which is obviously not as resilient as if the supplies were
originating from separate sources. It does, however, allow a bypass
supply to power the load in the event that the UPS has to be temporarily
taken out of service for maintenance, service or repair.
Resilience
of uninterruptible power supplies is what every power protection
project looks to maximise and dual input supplies are just one technique
that can be employed to ensure that loads can be kept operational even
in the face of system problems such as fault conditions, overload
shutdowns or power problems of any kind.
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